1. Field of the Invention
The present invention relates to a series of organic compounds which form a class of substances containing alkyl or alkoxy groups, which are only distinguished from one another by a NO.sub.2, Br or CN group and which, alone or in admixture, form a liquid crystal with a high negative dielectric anisotropy and which are, therefore, capable of producing high dynamic light scattering.
2. Description of the Prior Art
It is known that certain liquid crystals in the temperature range where they exist in the nematic mesomorphic phase, exhibit the effect of dynamic light scattering when they are subjected to a continuous or alternating electrical field. This effect is utilized in the electro-optical and visualization fields.
The performance characteristics of such liquid crystals are measured in terms of their operating temperature range, the contrast level of the material, and the service life when subjected to the voltage of a current.
It will be recalled that dynamic light scattering only appears for nematic compounds which possess negative dielectric anisotropy, i.e. compounds where the dielectric constant measured perpendicularly to the major axis of the molecule is greater than the dielectric constant measured parallel to the major axis. In order to obtain compounds of this type, a functional group which possesses a high dipole moment, for example, is connected to an elongated organic molecule in such a position that the resulting dipole moment is substantially perpendicular to the major axis of the molecule.
The series of elongated organic compounds are shown in French Pat. No. 2,181,943, filed Apr. 20, 1973, which have the general formula: ##STR2## wherein R.sup.5 and R.sup.8 are alkyl or alkoxy groups which have from one to twelve carbon atoms, or halogen atoms, and wherein R.sup.1 -R.sup.4, R.sup.6, and R.sup.7 are hydrogen, cyano, or halogen atoms or methyl or methoxy groups. Of most interest, at least in the context of the present invention, are the compounds in which R.sup.1 =R.sup.2 =R.sup.6 =R.sup.7 =H, R.sup.7 is Cl and R.sup.8 is C.sub.5 H.sub.11 O. However, the reference provides no information about the dielectric anisotropy of the compounds and no information about the synthesis of the compounds. In addition, only a few of all of the possible compounds within the scope of the generic formula are identified as mesomorphic over a given temperature range. For the compound of most pertinent interest to the present invention, a mesomorphic temperature range is described but no dielectric anisotropy value is given.
R. M. Goodwin et al, in "Research Disclosure", 131 pages 28-31 (March 1975) disclose liquid crystals having the formula: ##STR3## wherein R.sup.6 and R.sup.11 are alkyl or alkoxy and R.sup.7 -R.sup.10 are hydrogen, halogen, or cyano groups. The "linking group" can be a carbonyloxy group. The reference indicates that the compounds are liquid crystals which possess a positive zero frequency dielectric anisotropy and which exhibit a dielectric anisotropy inversion at increasing frequencies of an applied field in the audio frequency range. However, the compounds are not described as possessing high negative dielectric anisotropies.
A need, therefore, continues to exist for liquid crystal compounds which possess high negative dielectric anisotropies.